Upgrading video will always help OpenGL viewing one way or another. You may want to check system ram usage/CPU usage, at the point you see slowdowns, just in case the video is ok, but the system is slowing somewhere else. 

The amount of ram on that card should be fine unless textures are extreme(multi-layer etc.). Try zooming in to a scene and pan around, then from a distance. If the slowdown occurs at a distance alot more then up close, then a video upgrade would help more, but it also depends on the overall system speeds(bottlenecks) and scene complexity. Hope this helps.